As is known, radio frequency (RF) transmitters modulate baseband signals, such as analog voice or digital voice samples, onto an RF carrier, amplify the RF carder, and transmit the RF carrier, via an antenna, through the air as electromagnetic energy. The electromagnetic energy is subsequently received by a receiver's antenna, demodulated back to the baseband signal, and reconstructed into its original form by the receiver.
As is also known, many communication systems, such as cellular telephone and thinking systems, utilize spectrally efficient modulation techniques, such as quadrature amplitude modulation (QAM) and quaternary phase shift keying (QPSK), in a time division multiple access (TDMA) format. These spectrally efficient modulation techniques typically correlate the baseband signal to changes in RF carder amplitude and phase via a digital symbol constellation format. Since the spectrally efficient modulation techniques require variation of the RF carder amplitude, a linear class A or class AB amplifier must be used. If the amplifier is non-linear, it provides unwanted RF energy, or splatter, at frequencies adjacent to the RF carder. This splatter may subsequently interfere with two-way communications in process on the adjacent frequencies, or channels.
Linearity of a power amplifier is affected by the varying load impedances presented by the radio transmitter's antenna. Typically, an antenna is designed to provide a fixed load impedance, 50 ohms for example. However, due to the proximity of the antenna to highly reflective objects, such as automobiles or metal walls, the antenna impedance changes.
To minimize variations in power amplifier loading, transmitters generally incorporate isolators to provide a substantially constant load impedance to the amplifier. The isolator includes a circulator and a terminating impedance, which is typically 50 ohms. The circulator is a three-terminal device that provides unidirectional flow of the RF energy--i.e., from the amplifier to the antenna, and from the antenna to the terminating impedance. Therefore, the RF energy sourced by the amplifier is provided to the antenna and any RF energy reflected by the antenna is absorbed in the terminating impedance. In this manner, the isolator presents a constant impedance to the RF power amplifier irrespective of the antenna load impedance.
Although the isolator provides a constant load impedance, other factors--e.g., size, cost, and bandwidth limitations--typically inhibit the use of a universal isolator in mobile radios, portable radios, and cellular telephones. For example, a radio that operates at 132 MHz requires an isolator that has a volume of 8.19 cubic centimeters (0.5 cubic inches), weighs 227 grams (0.5 pounds), and costs at least $30/unit. As a result, an isolator puts undesired size, weight, and cost constraints on the design of such radios. Additionally, isolators have fixed bandwidths; therefore, multiple isolators may be required in transmitters that operate over a wide frequency range. This bandwidth limitation is most noticeable at lower RF carrier frequencies, such as VHF, where the allocated frequency band covers a large percentage bandwidth. Further, the isolator dissipates a considerable amount of RF energy when the antenna presents a highly reflective load impedance. This energy dissipation negatively impacts the net gain and efficiency of the radio transmitter.
To avoid the use of the isolator, existing frequency modulation (FM) transmitters, which employ nonlinear amplifiers, typically utilize protective feedback circuitry. The protective feedback circuitry monitors the voltage standing wave ratio (VSWR) at the nonlinear amplifier's output, and correspondingly reduces the mount of output power provided by the nonlinear amplifier to the antenna. This approach typically reduces the nonlinear amplifiers output power by a fixed mount when the VSWR exceeds a predetermined level. For example, when a 3:1 VSWR is detected at the nonlinear amplifier's output, the output power may be reduced by 3 dB. Although this approach works for nonlinear amplifiers, it does not include any provisions for maintaining amplifier linearity under high VSWR load conditions. Thus, this simple power reduction approach is not readily applicable for use in a linear amplifier.
Alternatively, a known method for detecting and correcting impedance mismatches may be used to obviate the use of an isolator in an FM transmitter. This method--as described in U.S. Pat. No. 4,704,573, entitled "Impedance Mismatch Detector" and assigned to Motorola, Inc.--allows impedance mismatches between the amplifier and the antenna to be measured and adaptively corrected during changes in operating conditions of the amplifier. Although this method provides a technique for electronically correcting poor loads presented to an amplifier, it is not readily adaptable for use in a linear transmitter since it does not provide means for changing the amplifier's load without influencing important linear performance parameters, such as adjacent channel splatter.
Therefore, a need exists for a method to enhance operating characteristics of a linear transmitter that operates under varying antenna loads without having to use an isolator.